Bend-Resistant Multimode Optical Fiber

ABSTRACT

A multimode optical fiber includes a central core surrounded by an outer cladding. The central core has a graded-index profile with respect to the outer cladding and an outer radius r 1  of between about 30 microns and 50 microns (e.g., between about 35 microns and 45 microns). The optical fiber also includes an inner cladding positioned between the central core and the outer cladding, and a depressed trench positioned between the inner cladding and the outer cladding. The multimode optical fiber exhibits reduced bending losses.

CROSS-REFERENCE TO PRIORITY APPLICATIONS

This application hereby claims the benefit of pending EuropeanApplication No. 11305328.4 for a “Multimode Optical Fiber with ImprovedBend Resistance” (filed Mar. 24, 2011, at the European Patent Office),which is hereby incorporated by reference in its entirety.

This application further claims the benefit of commonly assigned U.S.Provisional Patent Application Ser. No. 61/511,672 for a “MultimodeOptical Fiber with Improved Bend Resistance” (filed Jul. 26, 2011),which is hereby incorporated by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to the field of fiber optic transmissionand, more specifically, to a multimode optical fiber having reducedbending losses.

BACKGROUND

An optical fiber (i.e., a glass fiber typically surrounded by one ormore coating layers) conventionally includes an optical fiber core,which transmits and/or amplifies an optical signal, and an opticalcladding, which confines the optical signal within the core.Accordingly, the refractive index of the core n_(c) is typically greaterthan the refractive index of the optical cladding n_(g) (i.e.,n_(c)>n_(g)).

Multimode optical fibers are commonly used for short-distanceapplications requiring a broad bandwidth, such as local networks or LAN(local area network). The core of a multimode optical fiber typicallyhas a diameter of between about 50 microns and 62.5 microns, whereas thecore of a single-mode optical fiber typically has a diameter of betweenabout 6 microns and 9 microns. In a multimode optical fiber, for a givenwavelength, several optical modes are propagated simultaneously alongthe optical fiber.

Multimode optical fibers have been the subject of internationalstandardization under the ITU-T G.651.1 recommendations, which, inparticular, define criteria (e.g., bandwidth, numerical aperture, andcore diameter) that relate to the requirements for optical fibercompatibility. The ITU-T G.651.1 standard (July 2007) is herebyincorporated by reference in its entirety.

The numerical aperture (NA) of an optical fiber can be approximated bythe following equation:

NA=√{square root over (n _(c) ² −n _(g) ²)}

where n_(c) is the refractive index of the central core and n_(g) is therefractive index of the outer cladding (e.g., an outer opticalcladding).

For optical fibers, the refractive index profile is generally classifiedaccording to the graphical appearance of the function that associatesthe refractive index with the radius of the optical fiber.Conventionally, the distance r to the center of the optical fiber isshown on the x-axis, and the difference between the refractive index (atradius r) and the refractive index of the optical fiber's outer cladding(e.g., an outer optical cladding) is shown on the y-axis. The refractiveindex profile is referred to as a “step” profile, “trapezoidal” profile,“parabolic” profile, or “triangular” profile for graphs having therespective shapes of a step, a trapezoid, a parabola, or a triangle.These curves are generally representative of the optical fiber'stheoretical or set profile. Constraints in the manufacture of theoptical fiber, however, may result in a slightly different actualprofile.

For the same propagation medium (i.e., in a step-index multimode opticalfiber), the different modes have different group delay times. Thisdifference in group delay times results in a time lag (i.e., a delay)between the pulses propagating along different radial offsets of theoptical fiber. This delay causes a broadening of the resulting lightpulse. Broadening of the light pulse increases the risk of the pulsebeing superimposed onto a trailing pulse, which reduces the bandwidth(i.e., data rate) supported by the optical fiber. The bandwidth,therefore, is linked to the group delay time of the optical modespropagating in the multimode core of the optical fiber. Thus, toguarantee a broad bandwidth, it is desirable for the group delay timesof all the modes to be identical. Stated differently, the intermodaldispersion should be zero, or at least minimized, for a givenwavelength.

To reduce intermodal dispersion, the multimode optical fibers used intelecommunications generally have a core with a refractive index thatdecreases progressively from the center of the optical fiber to itsinterface with a cladding (i.e., an “alpha” core profile). Such anoptical fiber has been used for a number of years, and itscharacteristics have been described in “Multimode Theory of Graded-CoreFibers” by D. Gloge et al., Bell system Technical Journal 1973, pp.1563-1578, and summarized in “Comprehensive Theory of Dispersion inGraded-Index Optical Fibers” by G. Yabre, Journal of LightwaveTechnology, February 2000, Vol. 18, No. 2, pp. 166-177. Each of theabove-referenced articles is hereby incorporated by reference in itsentirety.

A graded-index profile (i.e., an alpha-index profile) can be describedby a relationship between the refractive index value n and the distancer from the center of the optical fiber according to the followingequation:

$\begin{matrix}{n = {n_{1}\sqrt{1 - {2{\Delta \left( \frac{r}{a} \right)}^{\alpha}}}}} & \left( {{Equation}\mspace{14mu} 1} \right)\end{matrix}$

wherein,

α≧1, and α is a non-dimensional parameter that is indicative of theshape of the index profile;

n₁ is the maximum refractive index of the optical fiber's core;

a is the radius of the optical fiber's core; and

$\begin{matrix}{\Delta = \frac{\left( {n_{1}^{2} - n_{0}^{2}} \right)}{2n_{1}^{2}}} & \left( {{Equation}\mspace{14mu} 2} \right)\end{matrix}$

where n₀ is the minimum refractive index of the multimode core, whichmay correspond to the refractive index of the outer cladding (most oftenmade of silica).

A multimode optical fiber with a graded index (i.e., an alpha profile)therefore has a core profile with a rotational symmetry such that alongany radial direction of the optical fiber the value of the refractiveindex decreases continuously from the center of the optical fiber's coreto its periphery. When a multimode light signal propagates in such agraded-index core, the different optical modes experience differingpropagation mediums (i.e., because of the varying refractive indices).This, in turn, affects the propagation speed of each optical modedifferently. Thus, by adjusting the value of the parameter α, it ispossible to obtain a group delay time that is virtually equal for all ofthe modes. Stated differently, the refractive index profile can bemodified to reduce or even eliminate intermodal dispersion.

Typically, multimode optical fibers with higher numerical apertures havelower macrobending losses (referred to hereafter as “bending losses”).

Conventional multimode optical fibers having a central core diameter ofmore than 50 microns are generally expected to provide sufficient bendresistance for many applications. Such exemplary optical fibers may havea central core diameter of 62.5 microns and a numerical aperture of0.275 or a central core diameter of 80 microns and a numerical apertureof 0.3.

Nevertheless, for tighter bend radii (e.g., 5 millimeters), such opticalfibers exhibit significant bending losses that may be critical for highspeed transmission (e.g., in compact consumer electronic devices).

International Publication No. WO 2010/036684, which is herebyincorporated by reference in its entirety, deals with large-core opticalfibers. The disclosed optical fibers, however, have a central-coreradius a and a relative refractive index difference Δ such that:

$\frac{\sqrt{2\Delta}}{a} < {5.1 \times 10^{- 3}\mspace{14mu} {{\mu m}^{- 1}.}}$

The central cores of the disclosed optical fibers fail to providereduced microbending losses, because, for a given Δ value, enlarging thecentral core will result in larger microbending losses. Furthermore, therelationships between the disclosed central-core radius a and therelative refractive index difference Δ lead to undesirably largemicrobending losses.

Therefore, a need exists for a multimode optical fiber having reducedbending losses and a central-core diameter of greater than 50 microns.

SUMMARY

Accordingly, in one aspect, the present invention embraces an opticalfiber that includes a central core surrounded by an outer cladding(e.g., an outer optical cladding). The outer cladding has a refractiveindex value n_(c1). The central core has an outer radius r₁, a maximumrefractive index value n₀, and a graded-index profile with respect tothe outer cladding. The central core also has a relative refractiveindex difference:

$\Delta = {\frac{n_{0}^{2} - n_{cl}^{2}}{2n_{0}^{2}}.}$

The central core's outer radius r₁ is typically between about 30 micronsand 50 microns (e.g., 35 microns to 50 microns). Typically, the centralcore's outer radius r₁ and relative refractive index difference Δsatisfy the following inequality:

$\frac{\sqrt{2\Delta}}{r_{1}} \geq {5.1 \times 10^{- 3}\mspace{14mu} {{\mu m}^{- 1}.}}$

An inner cladding is positioned between the central core and the outercladding (e.g., immediately surrounding the central core). The innercladding has an outer radius r₂ and a refractive index difference Δn₂with respect to the outer cladding. Typically, the difference r₂−r₁between the inner cladding's outer radius r₂ and the central core'souter radius r₁ is between about 1 micron and 3 microns.

A depressed trench is positioned between the inner cladding and theouter cladding (e.g., immediately surrounding the inner cladding). Thedepressed trench has an outer radius r₃ and a negative refractive indexdifference Δn₃ with respect to the outer cladding. Typically, thedifference r₃−r₂ between the depressed trench's outer radius r₃ and theinner cladding's outer radius r₂ is between about 3 microns and 7microns. The depressed trench's refractive index difference Δn₃ istypically between about −10×10⁻³ and −5×10⁻³.

In an exemplary embodiment, for two turns around a radius of curvatureof 5 millimeters at a wavelength of 850 nanometers, the optical fiberhas bending losses of less than about 0.3 dB.

In another exemplary embodiment, for two turns around a radius ofcurvature of 5 millimeters at a wavelength of 850 nanometers, theoptical fiber has bending losses of less than about 0.2 dB.

In yet another exemplary embodiment, the central core's outer radius r₁and relative refractive index difference Δ satisfy the followinginequality:

$\frac{\sqrt{2\Delta}}{r_{1}} \geq {5.4 \times 10^{- 3}\mspace{14mu} {{\mu m}^{- 1}.}}$

In yet another exemplary embodiment, the central core's outer radius r₁is between about 35 microns and 50 microns (e.g., between about 35microns and 45 microns).

In yet another exemplary embodiment, the central core's outer radius r₁is greater than about 36 microns (e.g., about 37 microns or greater).

In yet another exemplary embodiment, the central core's outer radius r₁is greater than about 38 microns (e.g., between about 39 microns and 41microns).

In yet another exemplary embodiment, the central core's outer diameter(i.e., 2r₁) is about 62.5 microns.

In yet another exemplary embodiment, the central core's outer diameter(i.e., 2r₁) is about 80 microns (i.e., outer radius r₁ is about 40microns).

In another aspect, the present invention embraces a multimode opticalsystem that includes an optical fiber in accordance with the foregoing.

The foregoing illustrative summary, as well as other exemplaryobjectives and/or advantages of the invention, and the manner in whichthe same are accomplished, are further explained within the followingdetailed description and its accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 graphically depicts the refractive index profile of an exemplaryoptical fiber according to the present invention.

FIG. 2 graphically depicts bending losses as a function of bendingradius (i.e., radius of curvature) for an exemplary optical fiberaccording to the present invention and a comparative optical fiber.

FIG. 3 graphically depicts bending losses as a function of bendingradius (i.e., radius of curvature) for another exemplary optical fiberaccording to the present invention and another comparative opticalfiber.

FIG. 4 depicts an optical-fiber cable undergoing a pinch test.

DETAILED DESCRIPTION

The present invention embraces a multimode optical fiber having reducedbending losses and a central-core diameter of greater than 50 microns.

FIG. 1 depicts the refractive index profile of an exemplary opticalfiber in accordance with the present invention. The exemplary opticalfiber is a multimode optical fiber that includes a central core (e.g.,an inner core) surrounded by an outer cladding (e.g., an outer opticalcladding).

As depicted, the exemplary optical fiber also includes an inner claddingpositioned between the central core and the outer cladding (e.g.,immediately surrounding the central core). A depressed trench ispositioned between the inner cladding and the outer optical cladding(e.g., immediately surrounding the inner cladding).

The central core has an outer radius r₁ that is typically between 30microns and 50 microns, more typically 35 microns or greater (e.g.,between about 39 microns and 41 microns). The central core also has agraded-index profile relative to the outer cladding. The central core'souter radius r₁ and relative refractive index difference Δ typicallysatisfy the following inequality:

$\frac{\sqrt{2\Delta}}{r_{1}} \geq {5.1 \times 10^{- 3}\mspace{14mu} {{\mu m}^{- 1}.}}$

The central core's relative refractive index difference Δ is defined bythe following equation:

${\Delta = \frac{n_{0}^{2} - n_{cl}^{2}}{2n_{0}^{2}}},$

where n₀ is the maximum refractive index value of the central core(typically corresponding to the refractive index value of the center ofthe central core) and n₀₁ is the refractive index value of the outercladding.

The optical fiber's inner cladding has an outer radius r₂. Typically,the difference r₂−r₁ between the inner cladding's outer radius r₂ andthe central core's outer radius r₁ (e.g., the radial distance betweenthe central core's outer radius and the inner radius of the depressedtrench) is between 1 micron and 3 microns (e.g., about 2 microns orless).

The depressed trench has an outer radius r₃ and a negative refractiveindex difference Δn₃ with respect to the outer cladding. Typically, thedifference r₃−r₂ between the depressed trench's outer radius r₃ and theinner cladding's outer radius r₂ (e.g., the width of the depressedtrench) is between 3 microns and 7 microns (e.g., about 5 microns orless, such as about 4 microns). The depressed trench's refractive indexdifference Δn₃ (e.g., the depth of the depressed trench) is typicallybetween −10×10⁻³ and −5×10⁻³.

The present multimode optical fiber provides excellent macrobendingperformance. For example, for two turns around a radius of curvature of5 millimeters at a wavelength of 850 nanometers, the optical fibertypically has bending losses of less than 0.3 dB (e.g., less than 0.2dB). In another exemplary embodiment, for one turn around a radius ofcurvature of 3 millimeters at a wavelength of 850 nanometers, theoptical fiber has bending losses of less than about 0.5 dB.

Exemplary multimode optical fibers in accordance with the presentinvention include a central core having an outer diameter of 62.5microns (i.e., 62.5-micron MMFs) and exhibit improved bend resistance.Additionally, exemplary multimode optical fibers include a central corehaving an outer diameter of 80 microns (i.e., 80-micron MMFs) andexhibit improved bend resistance.

As noted, exemplary multimode optical fibers include a central corehaving a graded-index profile relative to the outer cladding (i.e.,Graded-Index MultiMode Fibers or GI-MMFs). The modal structure ofGI-MMFs is as follows.

In multimode optical fibers with the following refractive index profile:

$\begin{matrix}{{n(r)} = \left\{ \begin{matrix}{{{n_{1} \cdot \sqrt{1 - {2{\Delta \cdot \left( \frac{r}{a} \right)^{\alpha}}}}}\mspace{14mu} {for}\mspace{14mu} r} \leq a} \\{{{{n_{1} \cdot \sqrt{1 - {2\Delta}}}\mspace{14mu} {for}\mspace{14mu} r} \geq a},}\end{matrix} \right.} & \left( {{Equation}\mspace{14mu} 3} \right)\end{matrix}$

where α is between about 1.9 and 2.1, Δ>1.2 percent and a>30 microns,the effective indices of the guided modes can be fairly approximated asfollows:

$\begin{matrix}{\beta = {{kn}_{1} \cdot \left\lbrack {1 - {2{\Delta \left( \frac{m}{M} \right)}^{\frac{\alpha}{\alpha + 2}}}} \right\rbrack^{\frac{1}{2}}}} & \left( {{Equation}\mspace{14mu} 4} \right)\end{matrix}$

where m is the order of the mode, and

${M = {{akn}_{1} \cdot \sqrt{\Delta \cdot \frac{\alpha}{\alpha + 2}}}},$

where

${k = \frac{2\pi}{\lambda}},$

and λ=850 nanometers.

For α approximately equal to 2, the spacing between modes is somewhatindependent from the mode order m and is only a function of the centralcore's radius a and the central core's height Δ:

$\begin{matrix}{{{\Delta\beta}} \propto \; \frac{\sqrt{2\Delta}}{a}} & \left( {{Equation}\mspace{14mu} 5} \right)\end{matrix}$

For a 50-micron GI-MMF with Δ=0.9 percent and a=26 microns (as athreshold) this mode spacing (i.e., Δβ is equal to 5.1×10⁻³ μm⁻¹.Acceptable tolerances with respect to core diameter (e.g., ±2.5 microns)and numerical aperture (e.g., ±0.015) may be taken into account in viewof constraints in the manufacturing process.

Without being bound to any particular theory, the present inventors havefound that mode spacing affects microbending losses. Indeed, consideringthe three examples of Table 1, larger values of √{square root over(2Δ)}/a (i.e., the mode-spacing criterion) correlate with lowermicrobending losses. In Table 1, the relative refractive indexdifference Δ is given as a percentage.

TABLE 1 Primary coating r₁ Criterion Microbending losses thickness Δ (%)(μm) (μm⁻¹) @ 850 nm (dB/km) (μm) 1.928 38.8 5.06 × 10⁻³ 27.5 27.9352.059 39.2 5.18 × 10⁻³ 20.3 27.93 2.601 38.4 5.94 × 10⁻³ 12.1 27.75

As shown in Table 1, an increase in the criterion from 5.06×10⁻³ to5.94×10⁻³ reduces microbending losses two-fold. Accordingly, exemplaryoptical fibers in accordance with the present invention have acentral-core outer radius r₁ and relative refractive index difference Δsuch that √{square root over (2Δ)}/r₁ is greater than or equal to5.1×10⁻³ μm⁻¹ (e.g., greater than or equal to 5.2×10⁻³ μm⁻¹), such asgreater than or equal to 5.4×10⁻³ μm⁻¹ (e.g., greater than or equal to5.6×10⁻³ μm⁻¹). Additionally, exemplary optical fibers may have acentral-core outer radius r₁ and relative refractive index difference Δsuch that √{square root over (2Δ)}/r₁ is greater than or equal to5.7×10⁻³ μm⁻¹ (e.g., greater than or equal to 5.9×10⁻³ μm⁻¹)

Microbending may be analyzed according to the IEC fixed-diametersandpaper drum test (i.e., IEC TR62221, Method B, 40-micron gradesandpaper), which provides a microbending stress situation that affectsmultimode fibers even at room temperature. The IEC TR62221microbending-sensitivity technical report and standard test procedures,including IEC TR62221, Method B (fixed-diameter sandpaper drum) andMethod D (basketweave), are hereby incorporated by reference in theirentirety.

To satisfy the foregoing relationship between the central core's outerradius r₁ and relative refractive index difference Δ, largercentral-core radii demand larger values for Δ, as illustrated in Table2. In Table 2, the relative refractive index difference Δ is given as apercentage.

TABLE 2 a (μm) Δ (%) NA 25 >0.81 >0.185 31.25 >1.27 >0.23240 >2.08 >0.297 50 >3.25 >0.373

Typically, macro-bending losses are lower for optical fibers with suchlarge numerical apertures. Furthermore, as illustrated in FIGS. 2 and 3,including a depressed trench between the central core and the outercladding further improves the macro-bending behavior of these large coreradii MMFs.

FIG. 2 graphically depicts bending losses as a function of bendingradius (i.e., radius of curvature) for an exemplary trench-assistedoptical fiber according to the present invention (i.e., Example 2) and acomparative optical fiber in accordance with the ITU-T G.651.1recommendations (i.e., Example 1).

FIG. 3 graphically depicts bending losses as a function of bendingradius (i.e., radius of curvature) for another exemplary trench-assistedoptical fiber according to the present invention (i.e., Example 4) andanother comparative optical fiber under a given Encircled Flux Launch(EFL) (i.e., Example 3).

The Encircled Flux (EF) in the optical fibers before bending fulfillsthe following criteria of Table 3:

TABLE 3 radius (μm) EF min EF max 10 0.2185 0.0635 15 0.38 0.2215 200.5249 0.4999

For 62.5-micron and 50-micron MFFs, the conditions on the EF formacrobending loss measurements are given by the ITU-T G.651.1recommendations and the EF templates within the IEC 61280-4-1 documents,which are hereby incorporated by reference in their entireties.

The comparative optical (Examples 1 and 3) and the exemplary opticalfibers (Examples 2 and 4) of FIGS. 2 and 3 correspond to optical fibershaving the parameters listed in Table 4.

TABLE 4 Bending losses Trench width Trench depth 2 turns 5 mm Examples a(μm) NA (μm) (×10⁻³) (dB) 1 31.25 0.27 none none 0.739 2 31.25 0.27 4 −80.138 3 41 0.31 none none 1.060 4 41 0.31 4 −8 0.253

As noted, Examples 2 and 4 are optical fibers according to the presentinvention that have a depressed trench of an appropriate width (i.e.,r₃−r₂) and depth (i.e., Δn₃). Examples 1 and 3 are comparative opticalfibers lacking a depressed trench. The bending losses are measured at awavelength of 850 nanometers.

As demonstrated by Table 4 and depicted in FIGS. 2 and 3, the depressedtrench facilitates the achievement of reduced bending losses. Indeed,the exemplary optical fibers exhibited bending losses that were morethan four times less than the bending losses of the comparative opticalfibers (i.e., a reduction to less than one-quarter of the comparativeoptical fibers' bending losses).

Advantages of large-core optical fibers having reduced bending lossesare further described in “80 μm-core graded-index MMF for consumerelectronic devices” by D. Molin, M. Bigot-Astruc, and P. Sillard,Optoelectronic Interconnects XII, Feb. 2, 2012, Proceedings of SPIE Vol.8267, which is hereby incorporated by reference in its entirety.

Exemplary optical fibers according to the present invention includedepressed trenches having particular volumes V₃. As used herein, thevolume V of a depressed trench is defined by the following equation:

V = 2 π × ∫_(r_(int))^(r_(ext))Δ%(r) × r × r

in which r_(int) and r_(ext) are the inner radius and outer radius ofthe depressed trench, respectively, and Δ % (r) is the depressedtrench's refractive index difference with respect to the outer claddingexpressed in terms of percentage. Those of ordinary skill in the artwill recognize that this equation can be used for both rectangular andnon-rectangular trenches.

If a depressed trench has a rectangular shape (i.e., a step indexprofile), the equation (above) can be simplified to the followingequation:

V=Δ%×π×(r _(ext) ² −r _(int) ²)

in which r_(ext) and r_(int) are the outer radius and inner radius ofthe depressed trench, respectively, and A % is the depressed trench'srefractive index difference with respect to the outer cladding expressedas a percentage.

In this regard, exemplary optical fibers include a depressed trenchhaving a volume V₃ of between about −1710%-μm² and −210%-μm² (e.g., avolume V₃ of about −830%-μm²). Typically, the depressed trench's volumeV₃ is about −790%-μm² or greater (e.g., a volume V₃ of about −750%-μm²or greater). More typically, the depressed trench's volume V₃ is about−710%-μm² or greater (e.g., a volume V₃ of about −670%-μm² or greater,such as about −635%-μm² or greater). In exemplary embodiments, thedepressed trench's volume V₃ is between about −750%-μm² and −450%-μm²(e.g., between about −628%-μm² and −565%-μm², such as about −600%-μm²).

An optical-fiber cable that includes one more optical fibers inaccordance with the present invention may be subjected to a pinch testas depicted in FIG. 4. During the pinch test the optical-fiber cable isfolded over itself so that the two cable portions on either side of thefold are substantially parallel to one another, thereby creating a pinchin the optical-fiber cable. The resulting folded cable has a maximumdiameter of about twice the cable's normal diameter. At the pinch point,the optical-fiber cable is flattened and has a diameter less than itsnormal diameter. Although the optical-fiber cable has a reduced diameterat and near the pinch point, the remainder of the cable retains itsnormal diameter. The cable is held in this pinched state for about 10minutes. After 10 minutes have elapsed and while the cable is held inthe pinched state, the attenuation of the optical fibers in the cable ismeasured (e.g., at room temperature).

During a pinch test at 850 nm, exemplary multimode optical fibersaccordance with the present invention (and contained within anoptical-fiber cable) typically have attenuation added losses of lessthan about 0.5 dB, more typically less than about 0.3 dB (e.g., lessthan about 0.25 dB, such as less than 0.20 dB).

Exemplary optical-fiber cables capable of excellent pinch-testperformance are disclosed in commonly assigned U.S. patent applicationSer. No. 13/401,026 for an Optical Fiber Interconnect Cable, which ishereby incorporated by reference in its entirety.

The present optical fibers may facilitate the reduction in overalloptical-fiber diameter. As will be appreciated by those having ordinaryskill in the art, a reduced-diameter optical fiber is cost-effective,requiring less raw material. Moreover, a reduced-diameter optical fiberrequires less deployment space (e.g., within a buffer tube and/or fiberoptic cable), thereby facilitating increased fiber count and/or reducedcable size.

Those having ordinary skill in the art will recognize that an opticalfiber with a primary coating (and an optional secondary coating and/orink layer) typically has an outer diameter of between about 235 micronsand about 265 microns (μm). The component glass fiber itself (i.e., theglass core and surrounding cladding layers) typically has a diameter ofabout 125 microns, such that the total coating thickness is typicallybetween about 55 microns and 70 microns.

With respect to the present optical fiber, the component glass fibertypically has an outer diameter of about 125 microns. With respect tothe optical fiber's surrounding coating layers, the primary coatingtypically has an outer diameter of between about 175 microns and about195 microns (i.e., a primary coating thickness of between about 25microns and 35 microns), and the secondary coating typically has anouter diameter of between about 235 microns and about 265 microns (i.e.,a secondary coating thickness of between about 20 microns and 45microns). Optionally, the present optical fiber may include an outermostink layer, which is typically between two and ten microns in thickness.

In one alternative embodiment, an optical fiber may possess a reduceddiameter (e.g., an outermost diameter between about 150 microns and 230microns). In this alternative optical fiber configuration, the thicknessof the primary coating and/or secondary coating is reduced, while thediameter of the component glass fiber is maintained at about 125microns. (Those having ordinary skill in the art will appreciate that,unless otherwise specified, diameter measurements refer to outerdiameters.)

By way of illustration, in such exemplary embodiments, the primarycoating layer may have an outer diameter of between about 135 micronsand about 175 microns (e.g., about 160 microns), typically less than 165microns (e.g., between about 135 microns and 150 microns), and usuallymore than 140 microns (e.g., between about 145 microns and 155 microns,such as about 150 microns).

Moreover, in such exemplary embodiments, the secondary coating layer mayhave an outer diameter of between about 150 microns and about 230microns (e.g., more than about 165 microns, such as 190-210 microns orso), typically between about 180 microns and 200 microns. In otherwords, the total diameter of the optical fiber is reduced to less thanabout 230 microns (e.g., between about 195 microns and 205 microns, andespecially about 200 microns). By way of further illustration, anoptical fiber may employ a secondary coating of about 197 microns at atolerance of +/−5 microns (i.e., a secondary-coating outer diameter ofbetween 192 microns to 202 microns). Typically, the secondary coatingwill retain a thickness of at least about 10 microns (e.g., an opticalfiber having a reduced thickness secondary coating of between 15 micronsand 25 microns).

In another alternative embodiment, the outer diameter of the componentglass fiber may be reduced to less than 125 microns (e.g., between about60 microns and 120 microns), perhaps between about 70 microns and 115microns (e.g., about 80-110 microns). This may be achieved, forinstance, by reducing the thickness of one or more cladding layers. Ascompared with the prior alternative embodiment, (i) the total diameterof the optical fiber may be reduced (i.e., the thickness of the primaryand secondary coatings are maintained in accordance with the prioralternative embodiment) or (ii) the respective thicknesses of theprimary and/or secondary coatings may be increased relative to the prioralternative embodiment (e.g., such that the total diameter of theoptical fiber might be maintained).

By way of illustration, with respect to the former, a component glassfiber having a diameter of between about 90 and 100 microns might becombined with a primary coating layer having an outer diameter ofbetween about 110 microns and 150 microns (e.g., about 125 microns) anda secondary coating layer having an outer diameter of between about 130microns and 190 microns (e.g., about 155 microns). With respect to thelatter, a component glass fiber having a diameter of between about 90and 100 microns might be combined with a primary coating layer having anouter diameter of between about 120 microns and 140 microns (e.g., about130 microns) and a secondary coating layer having an outer diameter ofbetween about 160 microns and 230 microns (e.g., about 195-200 microns).

Reducing the diameter of the component glass fiber might make theresulting optical fiber more susceptible to microbending attenuation.That said, the advantages of further reducing optical-fiber diametermight be worthwhile for some optical-fiber applications.

As noted, the present optical fibers may include one or more coatinglayers (e.g., a primary coating and a secondary coating). At least oneof the coating layers—typically the secondary coating—may be coloredand/or possess other markings to help identify individual fibers.Alternatively, a tertiary ink layer may surround the primary andsecondary coatings.

The present optical fibers may be manufactured by drawing from finalpreforms.

A final preform may be manufactured by providing a primary preform withan outer overcladding layer (i.e., an overcladding process). The outerovercladding layer typically consists of doped or undoped, natural orsynthetic, silica glass. Several methods are available for providing theouter overcladding layer.

In a first exemplary method, the outer overcladding layer may beprovided by depositing and vitrifying natural or synthetic silicaparticles on the outer periphery of the primary preform under theinfluence of heat. Such a process is known, for example, from U.S. Pat.Nos. 5,522,007, 5,194,714, 6,269,663, and 6,202,447, each of which ishereby incorporated by reference in its entirety.

In another exemplary method, a primary preform may be overcladded usinga silica sleeve tube, which may or may not be doped. This sleeve tubemay then be collapsed onto the primary preform.

In yet another exemplary method, an overcladding layer may be appliedvia an Outside Vapor Deposition (OVD) method. Here, a soot layer isfirst deposited on the outer periphery of a primary preform, and thenthe soot layer is vitrified to form glass.

The primary preforms may be manufactured via outside vapor depositiontechniques, such as Outside Vapor Deposition (OVD) and Vapor AxialDeposition (VAD). Alternatively, the primary preforms may bemanufactured via inside deposition techniques in which glass layers aredeposited on the inner surface of a substrate tube of doped or undopedsilica glass, such as Modified Chemical Vapor Deposition (MCVD), FurnaceChemical Vapor Deposition (FCVD), and Plasma Chemical Vapor Deposition(PCVD).

By way of example, the primary preforms may be manufactured using a PCVDprocess, which can precisely control the central core's gradientrefractive index profile.

A depressed trench, for instance, may be deposited on the inner surfaceof a substrate tube as part of the chemical vapor deposition process.More typically, a depressed trench may be manufactured either (i) byusing a fluorine-doped substrate tube as the starting point of theinternal deposition process for deposition of the gradient refractiveindex central core or (ii) by sleeving a fluorine-doped silica tube overthe gradient refractive index central core, which itself may be producedusing an outside deposition process (e.g., OVD or VAD). Accordingly, acomponent glass fiber manufactured from the resulting preform may have adepressed trench located at the periphery of its central core.

As noted, a primary preform may be manufactured via an inside depositionprocess using a fluorine-doped substrate tube. The resulting tubecontaining the deposited layers may be sleeved by one or more additionalfluorine-doped silica tubes so as to increase the thickness of adepressed trench, or to create a depressed trench having a varyingrefractive index over its width. Although not required, one or moreadditional sleeve tubes (e.g., fluorine-doped substrate tubes) may becollapsed onto the primary preform before an overcladding step iscarried out. The process of sleeving and collapsing is sometimesreferred to as jacketing and may be repeated to build several glasslayers on the outside of the primary preform.

The present optical fibers may be deployed in various structures, suchas those exemplary structures disclosed hereinafter.

For example, one or more of the present optical fibers may be enclosedwithin a buffer tube. For instance, optical fiber may be deployed ineither a single-fiber loose buffer tube or a multi-fiber loose buffertube. With respect to the latter, multiple optical fibers may be bundledor stranded within a buffer tube or other structure. In this regard,within a multi-fiber loose buffer tube, fiber sub-bundles may beseparated with binders (e.g., each fiber sub-bundle is enveloped in abinder). Moreover, fan-out tubing may be installed at the termination ofsuch loose buffer tubes to directly terminate loose buffered opticalfibers with field-installed connectors.

In other embodiments, the buffer tube may tightly surround the outermostoptical fiber coating (i.e., tight buffered fiber) or otherwise surroundthe outermost optical-fiber coating or ink layer to provide an exemplaryradial clearance of between about 50 and 100 microns (i.e., a semi-tightbuffered fiber).

With respect to the former tight buffered fiber, the buffering may beformed by coating the optical fiber with a curable composition (e.g., aUV-curable material) or a thermoplastic material. The outer diameter oftight buffer tubes, regardless of whether the buffer tube is formed froma curable or non-curable material, is typically less than about 1,000microns (e.g., either about 500 microns or about 900 microns).

With respect to the latter semi-tight buffered fiber, a lubricant may beincluded between the optical fiber and the buffer tube (e.g., to providea gliding layer).

As will be known by those having ordinary skill in the art, an exemplarybuffer tube enclosing optical fibers as disclosed herein may be formedof polyolefins (e.g., polyethylene or polypropylene), includingfluorinated polyolefins, polyesters (e.g., polybutylene terephthalate),polyamides (e.g., nylon), as well as other polymeric materials andblends. In general, a buffer tube may be formed of one or more layers.The layers may be homogeneous or include mixtures or blends of variousmaterials within each layer.

In this context, the buffer tube may be extruded (e.g., an extrudedpolymeric material) or pultruded (e.g., a pultruded, fiber-reinforcedplastic). By way of example, the buffer tube may include a material toprovide high temperature and chemical resistance (e.g., an aromaticmaterial or polysulfone material).

Although buffer tubes typically have a circular cross section, buffertubes alternatively may have an irregular or non-circular shape (e.g.,an oval or a trapezoidal cross-section).

Alternatively, one or more of the present optical fibers may simply besurrounded by an outer protective sheath or encapsulated within a sealedmetal tube. In either structure, no intermediate buffer tube isnecessarily required.

Multiple optical fibers as disclosed herein may be sandwiched,encapsulated, and/or edge bonded to form an optical fiber ribbon.Optical fiber ribbons can be divisible into subunits (e.g., atwelve-fiber ribbon that is splittable into six-fiber subunits).Moreover, a plurality of such optical fiber ribbons may be aggregated toform a ribbon stack, which can have various sizes and shapes.

For example, it is possible to form a rectangular ribbon stack or aribbon stack in which the uppermost and lowermost optical fiber ribbonshave fewer optical fibers than those toward the center of the stack.This construction may be useful to increase the density of opticalelements (e.g., optical fibers) within the buffer tube and/or cable.

In general, it is desirable to increase the filling of transmissionelements in buffer tubes or cables, subject to other constraints (e.g.,cable or mid-span attenuation). The optical elements themselves may bedesigned for increased packing density. For example, the optical fibermay possess modified properties, such as improved refractive-indexprofile, core or cladding dimensions, or primary-coating thicknessand/or modulus, to improve microbending and macrobendingcharacteristics.

By way of example, a rectangular ribbon stack may be formed with orwithout a central twist (i.e., a “primary twist”). Those having ordinaryskill in the art will appreciate that a ribbon stack is typicallymanufactured with rotational twist to allow the tube or cable to bendwithout placing excessive mechanical stress on the optical fibers duringwinding, installation, and use. In a structural variation, a twisted (oruntwisted) rectangular ribbon stack may be further formed into acoil-like configuration (e.g., a helix) or a wave-like configuration(e.g., a sinusoid). In other words, the ribbon stack may possess regular“secondary” deformations.

As will be known to those having ordinary skill in the art, such opticalfiber ribbons may be positioned within a buffer tube or othersurrounding structure, such as a buffer-tube-free cable. Subject tocertain restraints (e.g., attenuation), it is desirable to increase thedensity of elements such as optical fibers or optical fiber ribbonswithin buffer tubes and/or optical fiber cables.

A plurality of buffer tubes containing optical fibers (e.g., loose orribbonized fibers) may be positioned externally adjacent to and strandedaround a central strength member. This stranding can be accomplishedhelically in one direction, known as “S” or “Z” stranding, or viaReverse Oscillated Lay stranding, known as “S-Z” stranding. Strandingabout the central strength member reduces optical fiber strain whencable strain occurs during installation and use.

Those having ordinary skill in the art will understand the benefit ofminimizing fiber strain for both tensile cable strain and longitudinalcompressive cable strain during installation or operating conditions.

With respect to tensile cable strain, which may occur duringinstallation, the cable will become longer while the optical fibers canmigrate closer to the cable's neutral axis to reduce, if not eliminate,the strain being translated to the optical fibers. With respect tolongitudinal compressive strain, which may occur at low operatingtemperatures due to shrinkage of the cable components, the opticalfibers will migrate farther away from the cable's neutral axis toreduce, if not eliminate, the compressive strain being translated to theoptical fibers.

In a variation, two or more substantially concentric layers of buffertubes may be positioned around a central strength member. In a furthervariation, multiple stranding elements (e.g., multiple buffer tubesstranded around a strength member) may themselves be stranded aroundeach other or around a primary central strength member.

Alternatively, a plurality of buffer tubes containing optical fibers(e.g., loose or ribbonized fibers) may be simply placed externallyadjacent to the central strength member (i.e., the buffer tubes are notintentionally stranded or arranged around the central strength member ina particular manner and run substantially parallel to the centralstrength member).

Alternatively still, the present optical fibers may be positioned withina central buffer tube (i.e., the central buffer tube cable has a centralbuffer tube rather than a central strength member). Such a centralbuffer tube cable may position strength members elsewhere. For instance,metallic or non-metallic (e.g., GRP) strength members may be positionedwithin the cable sheath itself, and/or one or more layers ofhigh-strength yarns (e.g., aramid or non-aramid yarns) may be positionedparallel to or wrapped (e.g., contrahelically) around the central buffertube (i.e., within the cable's interior space). As will be understood bythose having ordinary skill in the art, such strength yarns providetensile strength to fiber optic cables. Likewise, strength members canbe included within the buffer tube's casing.

Strength yarns may be coated with a lubricant (e.g., fluoropolymers),which may reduce unwanted attenuation in fiber optic cables (e.g.,rectangular, flat ribbon cables or round, loose tube cables) that aresubjected to relatively tight bends (i.e., a low bend radius). Moreover,the presence of a lubricant on strength yarns (e.g., aramid strengthyarns) may facilitate removal of the cable jacketing by reducingunwanted bonding between the strength yarns and the surrounding cablejacket.

In other embodiments, the optical fibers may be placed within a slottedcore cable. In a slotted core cable, optical fibers, individually or asa fiber ribbon, may be placed within pre-shaped helical grooves (i.e.,channels) on the surface of a central strength member, thereby forming aslotted core unit. The slotted core unit may be enclosed by a buffertube. One or more of such slotted core units may be placed within aslotted core cable. For example, a plurality of slotted core units maybe helically stranded around a central strength member.

Alternatively, the optical fibers may also be stranded in a maxitubecable design, whereby the optical fibers are stranded around themselveswithin a large multi-fiber loose buffer tube rather than around acentral strength member. In other words, the large multi-fiber loosebuffer tube is centrally positioned within the maxitube cable. Forexample, such maxitube cables may be deployed in optical ground wires(OPGW).

In another cabling embodiment, multiple buffer tubes may be strandedaround themselves without the presence of a central member. Thesestranded buffer tubes may be surrounded by a protective tube. Theprotective tube may serve as the outer casing of the fiber optic cableor may be further surrounded by an outer sheath. The protective tube mayeither tightly surround or loosely surround the stranded buffer tubes.

As will be known to those having ordinary skill in the art, additionalelements may be included within a cable core. For example, copper cablesor other active, transmission elements may be stranded or otherwisebundled within the cable sheath. Passive elements may also be placedwithin the cable core, such as between the interior walls of the buffertubes and the enclosed optical fibers. Alternatively and by way ofexample, passive elements may be placed outside the buffer tubes betweenthe respective exterior walls of the buffer tubes and the interior wallof the cable jacket, or within the interior space of a buffer-tube-freecable.

For example, yarns, nonwovens, fabrics (e.g., tapes), foams, or othermaterials containing water-swellable material and/or coated withwater-swellable materials (e.g., including super absorbent polymers(SAPs), such as SAP powder) may be employed to provide water blockingand/or to couple the optical fibers to the surrounding buffer tubeand/or cable jacketing (e.g., via adhesion, friction, and/orcompression). Exemplary water-swellable elements are disclosed incommonly assigned U.S. Pat. No. 7,515,795 for a Water-Swellable Tape,Adhesive-Backed for Coupling When Used Inside a Buffer Tube, which ishereby incorporated by reference in its entirety.

Moreover, an adhesive (e.g., a hot-melt adhesive or curable adhesive,such as a silicone acrylate cross-linked by exposure to actinicradiation) may be provided on one or more passive elements (e.g.,water-swellable material) to bond the elements to the buffer tube. Anadhesive material may also be used to bond the water-swellable elementto optical fibers within the buffer tube. Exemplary arrangements of suchelements are disclosed in commonly assigned U.S. Pat. No. 7,599,589 fora Gel-Free Buffer Tube with Adhesively Coupled Optical Element, which ishereby incorporated by reference in its entirety.

The buffer tubes (or buffer-tube-free cables) may also contain athixotropic composition (e.g., grease or grease-like gels) between theoptical fibers and the interior walls of the buffer tubes. For example,filling the free space inside a buffer tube with water-blocking,petroleum-based filling grease helps to block the ingress of water.Further, the thixotropic filling grease mechanically (i.e., viscously)couples the optical fibers to the surrounding buffer tube.

Such thixotropic filling greases are relatively heavy and messy, therebyhindering connection and splicing operations. Thus, the present opticalfibers may be deployed in dry cable structures (i.e., grease-free buffertubes).

Exemplary buffer tube structures that are free from thixotropic fillinggreases are disclosed in commonly assigned U.S. Pat. No. 7,724,998 for aCoupling Composition for Optical Fiber Cables (Parris et al.), which ishereby incorporated by reference in its entirety. Such buffer tubesemploy coupling compositions formed from a blend of high-molecularweight elastomeric polymers (e.g., about 35 weight percent or less) andoils (e.g., about 65 weight percent or more) that flow at lowtemperatures. Unlike thixotropic filling greases, the couplingcomposition (e.g., employed as a cohesive gel or foam) is typically dryand, therefore, less messy during splicing.

As will be understood by those having ordinary skill in the art, a cableenclosing optical fibers as disclosed herein may have a sheath formedfrom various materials in various designs. Cable sheathing may be formedfrom polymeric materials such as, for example, polyethylene,polypropylene, polyvinyl chloride (PVC), polyamides (e.g., nylon),polyester (e.g., PBT), fluorinated plastics (e.g., perfluorethylenepropylene, polyvinyl fluoride, or polyvinylidene difluoride), andethylene vinyl acetate. The sheath and/or buffer tube materials may alsocontain other additives, such as nucleating agents, flame-retardants,smoke-retardants, antioxidants, UV absorbers, and/or plasticizers.

The cable sheathing may be a single jacket formed from a dielectricmaterial (e.g., non-conducting polymers), with or without supplementalstructural components that may be used to improve the protection (e.g.,from rodents) and strength provided by the cable sheath. For example,one or more layers of metallic (e.g., steel) tape, along with one ormore dielectric jackets, may form the cable sheathing. Metallic orfiberglass reinforcing rods (e.g., GRP) may also be incorporated intothe sheath. In addition, aramid, fiberglass, or polyester yarns may beemployed under the various sheath materials (e.g., between the cablesheath and the cable core), and/or ripcords may be positioned, forexample, within the cable sheath.

Similar to buffer tubes, optical fiber cable sheaths typically have acircular cross section, but cable sheaths alternatively may have anirregular or non-circular shape (e.g., an oval, trapezoidal, or flatcross-section).

By way of example, the present optical fiber may be incorporated intosingle-fiber drop cables, such as those employed for Multiple DwellingUnit (MDU) applications. In such deployments, the cable jacketing mustexhibit crush resistance, abrasion resistance, puncture resistance,thermal stability, and fire resistance as required by building codes. Anexemplary material for such cable jackets is thermally stable,flame-retardant polyurethane (PUR), which mechanically protects theoptical fibers yet is sufficiently flexible to facilitate easy MDUinstallations. Alternatively, a flame-retardant polyolefin or polyvinylchloride sheath may be used.

In general, and as will be known to those having ordinary skill in theart, a strength member is typically in the form of a rod orbraided/helically wound wires or fibers, though other configurationswill be within the knowledge of those having ordinary skill in the art.

Optical fiber cables containing optical fibers as disclosed may bevariously deployed, including as drop cables, distribution cables,feeder cables, trunk cables, and stub cables, each of which may havevarying operational requirements (e.g., temperature range, crushresistance, UV resistance, and minimum bend radius).

Such optical fiber cables may be installed within ducts, microducts,plenums, or risers. By way of example, an optical fiber cable may beinstalled in an existing duct or microduct by pulling or blowing (e.g.,using compressed air). An exemplary cable installation method isdisclosed in commonly assigned U.S. Pat. No. 7,574,095 for aCommunication Cable Assembly and Installation Method, (Lock et al.), andU.S. Pat. No. 7,665,902 for a Modified Pre-Ferrulized CommunicationCable Assembly and Installation Method, (Griffioen et al.), each ofwhich is incorporated by reference in its entirety.

As noted, buffer tubes containing optical fibers (e.g., loose orribbonized fibers) may be stranded (e.g., around a central strengthmember). In such configurations, an optical fiber cable's protectiveouter sheath may have a textured outer surface that periodically varieslengthwise along the cable in a manner that replicates the strandedshape of the underlying buffer tubes. The textured profile of theprotective outer sheath can improve the blowing performance of theoptical fiber cable. The textured surface reduces the contact surfacebetween the cable and the duct or microduct and increases the frictionbetween the blowing medium (e.g., air) and the cable. The protectiveouter sheath may be made of a low coefficient-of-friction material,which can facilitate blown installation. Moreover, the protective outersheath can be provided with a lubricant to further facilitate blowninstallation.

In general, to achieve satisfactory long-distance blowing performance(e.g., between about 3,000 to 5,000 feet or more), the outer cablediameter of an optical fiber cable should be no more than about 70 to 80percent of the duct's or microduct's inner diameter.

Compressed air may also be used to install optical fibers in an airblown fiber system. In an air blown fiber system, a network of unfilledcables or microducts is installed prior to the installation of opticalfibers. Optical fibers may subsequently be blown into the installedcables as necessary to support the network's varying requirements.

Moreover, the optical fiber cables may be directly buried in the groundor, as an aerial cable, suspended from a pole or pylon. An aerial cablemay be self-supporting, or secured or lashed to a support (e.g.,messenger wire or another cable). Exemplary aerial fiber optic cablesinclude overhead ground wires (OPGW), all-dielectric self-supportingcables (ADSS), all dielectric lash cables (AD-Lash), and figure-eightcables, each of which is well understood by those having ordinary skillin the art. Figure-eight cables and other designs can be directly buriedor installed into ducts, and may optionally include a toning element,such as a metallic wire, so that they can be found with a metaldetector.

In addition, although the optical fibers may be further protected by anouter cable sheath, the optical fiber itself may be further reinforcedso that the optical fiber may be included within a breakout cable, whichallows for the individual routing of individual optical fibers.

To effectively employ the present optical fibers in a transmissionsystem, connections are required at various points in the network.Optical fiber connections are typically made by fusion splicing,mechanical splicing, or mechanical connectors.

The mating ends of connectors can be installed to the optical fiber endseither in the field (e.g., at the network location) or in a factoryprior to installation into the network. The ends of the connectors aremated in the field in order to connect the optical fibers together orconnect the optical fibers to the passive or active components. Forexample, certain optical fiber cable assemblies (e.g., furcationassemblies) can separate and convey individual optical fibers from amultiple optical fiber cable to connectors in a protective manner.

The deployment of such optical fiber cables may include supplementalequipment, which itself may employ the present optical fiber aspreviously disclosed. For instance, an amplifier may be included toimprove optical signals. Dispersion compensating modules may beinstalled to reduce the effects of chromatic dispersion and polarizationmode dispersion. Splice boxes, pedestals, and distribution frames, whichmay be protected by an enclosure, may likewise be included. Additionalelements include, for example, remote terminal switches, optical networkunits, optical splitters, and central office switches.

A cable containing the present optical fibers may be deployed for use ina communication system (e.g., networking or telecommunications). Acommunication system may include fiber optic cable architecture such asfiber-to-the-node (FTTN), fiber-to-the-telecommunications enclosure(FTTE), fiber-to-the-curb (FITC), fiber-to-the-building (FTTB), andfiber-to-the-home (FTTH), as well as long-haul or metro architecture.Moreover, an optical module or a storage box that includes a housing mayreceive a wound portion of the optical fiber disclosed herein. By way ofexample, the optical fiber may be wound around a bending radius of lessthan about 15 millimeters (e.g., 10 millimeters or less, such as about 5millimeters) in the optical module or the storage box.

Moreover, present optical fibers may be used in other applications,including, without limitation, fiber optic sensors or illuminationapplications (e.g., lighting).

The present optical fibers may include Fiber Bragg Grating (FBG). Aswill be known by those having ordinary skill in the art, FBG is aperiodic or aperiodic variation in the refractive index of an opticalfiber core and/or cladding. This variation in the refractive indexresults in a range of wavelengths (e.g., a narrow range) being reflectedrather than transmitted, with maximum reflectivity occurring at theBragg wavelength.

Fiber Bragg Grating is commonly written into an optical fiber byexposing the optical fiber to an intense source of ultraviolet light(e.g., a UV laser). In this respect, UV photons may have enough energyto break molecular bonds within an optical fiber, which alters thestructure of the optical fiber, thereby increasing the optical fiber'srefractive index. Moreover, dopants (e.g., boron or germanium) and/orhydrogen loading can be employed to increase photosensitivity.

In order to expose a coated glass fiber to UV light for the creation ofFBG, the coating may be removed. Alternatively, coatings that aretransparent at the particular UV wavelengths (e.g., the UV wavelengthsemitted by a UV laser to write FBG) may be employed to render coatingremoval unnecessary. In addition, silicone, polyimide, acrylate, or PFCBcoatings, for instance, may be employed for high-temperatureapplications.

A particular FBG pattern may be created by employing (i) a photomaskplaced between the UV light source and the optical fiber, (ii)interference between multiple UV light beams, which interfere with eachother in accordance with the desired FBG pattern (e.g., a uniform,chirped, or titled pattern), or (iii) a narrow UV light beam forcreating individual variations. The FBG structure may have, for example,a uniform positive-only index change, a Gaussian-apodized index change,a raised-cosine-apodized index change, or a discrete phase-shift indexchange. Multiple FBG patterns may be combined on a single optical fiber.

Optical fibers having FBG may be employed in various sensingapplications (e.g., for detecting vibration, temperature, pressure,moisture, or movement). In this respect, changes in the optical fiber(e.g., a change in temperature) result in a shift in the Braggwavelength, which is measured by a sensor. FBG may be used to identify aparticular optical fiber (e.g., if the optical fiber is broken intopieces).

Fiber Bragg Grating may also be used in various active or passivecommunication components (e.g., wavelength-selective filters,multiplexers, demultiplexers, Mach-Zehnder interferometers, distributedBragg reflector lasers, pump/laser stabilizers, and supervisorychannels).

To supplement the present disclosure, this application incorporatesentirely by reference the following commonly assigned patents, patentapplication publications, and patent applications: U.S. Pat. No.4,838,643 for a Single Mode Bend Insensitive Fiber for Use in FiberOptic Guidance Applications (Hodges et al.); U.S. Pat. No. 7,623,747 fora Single Mode Optical Fiber (de Montmorillon et al.); U.S. Pat. No.7,587,111 for a Single-Mode Optical Fiber (de Montmorillon et al.); U.S.Pat. No. 7,356,234 for a Chromatic Dispersion Compensating Fiber (deMontmorillon et al.); U.S. Pat. No. 7,483,613 for a Chromatic DispersionCompensating Fiber (Bigot-Astruc et al.); U.S. Pat. No. 7,526,177 for aFluorine-Doped Optical Fiber (Matthijsse et al.); U.S. Pat. No.7,555,186 for an Optical Fiber (Flammer et al.); U.S. Pat. No. 8,055,111for a Dispersion-Shifted Optical Fiber (Sillard et al.); U.S. Pat. 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To supplement the present disclosure, this application furtherincorporates entirely by reference the following commonly assignedpatents, patent application publications, and patent applications: U.S.Pat. No. 5,574,816 for Polypropylene-Polyethylene Copolymer Buffer Tubesfor Optical Fiber Cables and Method for Making the Same; U.S. Pat. No.5,717,805 for Stress Concentrations in an Optical Fiber Ribbon toFacilitate Separation of Ribbon Matrix Material; U.S. Pat. No. 5,761,362for Polypropylene-Polyethylene Copolymer Buffer Tubes for Optical FiberCables and Method for Making the Same; U.S. Pat. No. 5,911,023 forPolyolefin Materials Suitable for Optical Fiber Cable Components; U.S.Pat. No. 5,982,968 for Stress Concentrations in an Optical Fiber Ribbonto Facilitate Separation of Ribbon Matrix Material; U.S. Pat. No.6,035,087 for an Optical Unit for Fiber Optic Cables; U.S. Pat. No.6,066,397 for Polypropylene Filler Rods for Optical Fiber CommunicationsCables; U.S. Pat. No. 6,175,677 for an Optical Fiber Multi-Ribbon andMethod for Making the Same; U.S. Pat. No. 6,085,009 for Water BlockingGels Compatible with Polyolefin Optical Fiber Cable Buffer Tubes andCables Made Therewith; U.S. Pat. No. 6,215,931 for FlexibleThermoplastic Polyolefin Elastomers for Buffering Transmission Elementsin a Telecommunications Cable; U.S. Pat. No. 6,134,363 for a Method forAccessing Optical Fibers in the Midspan Region of an Optical FiberCable; U.S. Pat. No. 6,381,390 for a Color-Coded Optical Fiber Ribbonand Die for Making the Same; U.S. Pat. No. 6,181,857 for a Method forAccessing Optical Fibers Contained in a Sheath; U.S. Pat. No. 6,314,224for a Thick-Walled Cable Jacket with Non-Circular Cavity Cross Section;U.S. Pat. No. 6,334,016 for an Optical Fiber Ribbon Matrix MaterialHaving Optimal Handling Characteristics; U.S. Pat. No. 6,321,012 for anOptical Fiber Having Water Swellable Material for Identifying Groupingof Fiber Groups; U.S. Pat. No. 6,321,014 for a Method for ManufacturingOptical Fiber Ribbon; U.S. Pat. No. 6,210,802 for Polypropylene FillerRods for Optical Fiber Communications Cables; U.S. Pat. No. 6,493,491for an Optical Drop Cable for Aerial Installation; U.S. Pat. No.7,346,244 for a Coated Central Strength Member for Fiber Optic Cableswith Reduced Shrinkage; U.S. Pat. No. 6,658,184 for a Protective Skinfor Optical Fibers; U.S. Pat. No. 6,603,908 for a Buffer Tube thatResults in Easy Access to and Low Attenuation of Fibers Disposed WithinBuffer Tube; U.S. Pat. No. 7,045,010 for an Applicator for High-SpeedGel Buffering of Flextube Optical Fiber Bundles; U.S. Pat. No. 6,749,446for an Optical Fiber Cable with Cushion Members Protecting Optical FiberRibbon Stack; U.S. Pat. No. 6,922,515 for a Method and Apparatus toReduce Variation of Excess Fiber Length in Buffer Tubes of Fiber OpticCables; U.S. Pat. No. 6,618,538 for a Method and Apparatus to ReduceVariation of Excess Fiber Length in Buffer Tubes of Fiber Optic Cables;U.S. Pat. No. 7,322,122 for a Method and Apparatus for Curing a FiberHaving at Least Two Fiber Coating Curing Stages; U.S. Pat. No. 6,912,347for an Optimized Fiber Optic Cable Suitable for Microduct BlownInstallation; U.S. Pat. No. 6,941,049 for a Fiber Optic Cable Having NoRigid Strength Members and a Reduced Coefficient of Thermal Expansion;U.S. Pat. No. 7,162,128 for Use of Buffer Tube Coupling Coil to PreventFiber Retraction; U.S. Pat. No. 7,515,795 for a Water-Swellable Tape,Adhesive-Backed for Coupling When Used Inside a Buffer Tube (Overton etal.); U.S. Patent Application Publication No. 2008/0292262 for aGrease-Free Buffer Optical Fiber Buffer Tube Construction Utilizing aWater-Swellable, Texturized Yarn (Overton et al.); European PatentApplication Publication No. 1,921,478 A1, for a TelecommunicationOptical Fiber Cable (Tatat et al.); U.S. Pat. No. 7,702,204 for a Methodfor Manufacturing an Optical Fiber Preform (Gonnet et al.); U.S. Pat.No. 7,570,852 for an Optical Fiber Cable Suited for Blown Installationor Pushing Installation in Microducts of Small Diameter (Nothofer etal.); U.S. Pat. No. 7,646,954 for an Optical Fiber TelecommunicationsCable (Tatat); U.S. Pat. No. 7,599,589 for a Gel-Free Buffer Tube withAdhesively Coupled Optical Element (Overton et al.); U.S. Pat. No.7,567,739 for a Fiber Optic Cable Having a Water-Swellable Element(Overton); U.S. Pat. No. 7,817,891 for a Method for Accessing OpticalFibers within a Telecommunication Cable (Lavenne et al.); U.S. Pat. No.7,639,915 for an Optical Fiber Cable Having a Deformable CouplingElement (Parris et al.); U.S. Pat. No. 7,646,952 for an Optical FiberCable Having Raised Coupling Supports (Parris); U.S. Pat. No. 7,724,998for a Coupling Composition for Optical Fiber Cables (Parris et al.);U.S. Patent Application Publication No. US2009/0214167 A1 for a BufferTube with Hollow Channels, (Lookadoo et al.); U.S. Patent ApplicationPublication No. US2009/0297107 A1 for an Optical Fiber TelecommunicationCable, filed May 15, 2009, (Tatat); U.S. Patent Application PublicationNo. US2009/0279833 A1 for a Buffer Tube with Adhesively Coupled OpticalFibers and/or Water-Swellable Element, filed Jul. 21, 2009, (Overton etal.); U.S. Patent Application Publication No. US2010/0092135 A1 for anOptical Fiber Cable Assembly, filed Sep. 10, 2009, (Barker et al.); U.S.Pat. No. 7,974,507 A1 for a High-Fiber-Density Optical Fiber Cable(Louie et al.); U.S. Pat. No. 7,970,247 for a Buffer Tubes for Mid-SpanStorage (Barker); U.S. Pat. No. 8,081,853 for Single-Fiber Drop Cablesfor MDU Deployments, filed Nov. 9, 2009, (Overton); U.S. Pat. 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No. 8,031,997 for aReduced-Diameter, Easy-Access Loose Tube Cable, filed Nov. 10, 2009,(Overton); U.S. Patent Application Publication No. US2010/0154479 A1 fora Method and Device for Manufacturing an Optical Preform, filed Dec. 19,2009, (Milicevic et al.); U.S. Patent Application Publication No. US2010/0166375 for a Perforated Water-Blocking Element, filed Dec. 29,2009, (Parris); U.S. Patent Application Publication No. US2010/0183821A1 for a UVLED Apparatus for Curing Glass-Fiber Coatings, filed Dec. 30,2009, (Hartsuiker et al.); U.S. Patent Application Publication No.US2010/0202741 A1 for a Central-Tube Cable with High-ConductivityConductors Encapsulated with High-Dielectric-Strength Insulation, filedFeb. 4, 2010, (Ryan et al.); U.S. Patent Application Publication No.US2010/0215328 A1 for a Cable Having Lubricated, Extractable Elements,filed Feb. 23, 2010, (Tatat et al.); U.S. Patent Application PublicationNo. US2011/0026889 A1 for a Tight-Buffered Optical Fiber Unit HavingImproved Accessibility, filed Jul. 26, 2010, (Risch et al.); U.S. PatentApplication Publication No. US2011/0064371 A1 for Methods and Devicesfor Cable Insertion into Latched Conduit, filed Sep. 14, 2010,(Leatherman et al.); U.S. Patent Application Publication No.2011/0069932 A1 for a High-Fiber-Density Optical-Fiber Cable, filed Oct.19, 2010, (Overton et al.); U.S. Patent Application Publication No.2011/0091171 A1 for an Optical-Fiber Cable Having High Fiber Count andHigh Fiber Density, filed Oct. 19, 2010, (Tatat et al.); U.S. PatentApplication Publication No. 2011/0176782 A1 for a Water-SolubleWater-Blocking Element, filed Jan. 19, 2011, (Parris); U.S. PatentApplication Publication No. 2011/0268400 A1 for a Data-Center Cable,filed Apr. 28, 2011, (Louie et al.); U.S. Patent Application PublicationNo. 2011/0268398 A1 for a Bundled Fiber Optic Cables, filed May 3, 2011,(Quinn et al.); U.S. Patent Application Publication No. 2011/0287195 A1for a Curing Apparatus Employing Angled UVLEDs, filed May 19, 2011,(Molin); U.S. Patent Application Publication No. 2012/0009358 for aCuring Apparatus Having UV Sources That Emit Differing Ranges of UVRadiation, filed Jun. 3, 2011, (Gharbi et al.); U.S. Patent ApplicationPublication No. 2012/0014652 A1 for a Adhesively Coupled Optical Fibersand Enclosing Tape, filed Jul. 13, 2011, (Parris); U.S. PatentApplication Publication No. 2012/0040105 A1 for a Method and ApparatusProviding Increased UVLED Intensity, filed Aug. 10, 2011, (Overton);U.S. Patent Application Publication No. 2012/0057833 A1 for anOptical-Fiber Module Having Improved Accessibility, filed Aug. 31, 2011,(Tatat); U.S. patent application Ser. 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In the specification and/or figures, typical embodiments of theinvention have been disclosed. The present invention is not limited tosuch exemplary embodiments. The use of the term “and/or” includes anyand all combinations of one or more of the associated listed items. Thefigures are schematic representations and so are not necessarily drawnto scale. Unless otherwise noted, specific terms have been used in ageneric and descriptive sense and not for purposes of limitation.

1. A multimode optical fiber, comprising: a central core surrounded byan outer optical cladding having a refractive index value n_(c1), saidcentral core having (i) an outer radius r₁ of about 35 microns orgreater, (ii) a maximum refractive index value n₀, (iii) a graded-indexprofile with respect to said outer optical cladding, and (iv) a relativerefractive index difference:${\Delta = \frac{n_{0}^{2} - n_{cl}^{2}}{2n_{0}^{2}}};$ an innercladding positioned between said central core and said outer cladding,said inner cladding having an outer radius r₂; and a depressed trenchpositioned between said inner cladding and said outer optical cladding,said depressed trench having an outer radius r₃, a refractive indexdifference Δn₃ with respect to said outer optical cladding, and a volumeV₃; wherein $\frac{\sqrt{2\Delta}}{r_{1}}$ is about 5.1×10⁻³ μm⁻¹ orgreater.
 2. The optical fiber according to claim 1, wherein$\frac{\sqrt{2\Delta}}{r_{1}}$ is about 5.7×10⁻³ μm⁻¹ or greater. 3.The optical fiber according to claim 1, wherein said depressed trench'svolume V₃ is −635%-μm² or greater.
 4. The optical fiber according toclaim 1, wherein: said central core's outer radius r₁ is 36 microns orgreater; and said depressed trench's volume V₃ is −670%-μm² or greater.5. The optical fiber according to claim 1, wherein: said central core'souter radius r₁ is 37 microns or greater; and$\frac{\sqrt{2\Delta}}{r_{1}}$ is about 5.4×10⁻³ μm⁻¹ or greater. 6.The optical fiber according to claim 1, wherein: said central core'souter radius r₁ is 38 microns or greater; and said depressed trench'svolume V₃ is −750%-μm² or greater.
 7. The optical fiber according toclaim 1, wherein: said central core's outer radius r₁ is 39 microns orgreater; and $\frac{\sqrt{2\Delta}}{r_{1}}$ is about 5.2×10⁻³ μm⁻¹ orgreater.
 8. The optical fiber according to claim 1, wherein: saidcentral core's outer radius r₁ is 39 microns or greater; and saiddepressed trench's volume V₃ is −790%-μm² or greater.
 9. The opticalfiber according to claim 1, wherein said central core's outer radius r₁is 40 microns or greater.
 10. A multimode optical fiber, comprising: acentral core surrounded by an outer optical cladding having a refractiveindex value n_(c1), said central core having (i) an outer radius r₁ ofbetween about 35 microns and 50 microns, (ii) a maximum refractive indexvalue n₀, (iii) a graded-index profile with respect to said outeroptical cladding, and (iv) a relative refractive index difference:${\Delta = \frac{n_{0}^{2} - n_{cl}^{2}}{2n_{0}^{2}}};$ an innercladding positioned between said central core and said outer cladding,said inner cladding having an outer radius r₂; and a depressed trenchpositioned between said inner cladding and said outer optical cladding,said depressed trench having an outer radius r₃, a refractive indexdifference Δn₃ with respect to said outer optical cladding of betweenabout −10×10⁻³ and −5×10⁻³, and a volume V₃; wherein$\frac{\sqrt{2\Delta}}{r_{1}}$ is about 5.1×10⁻³ μm⁻¹ or greater;wherein the difference r₂−r₁ between said inner cladding's outer radiusr₂ and said central core's outer radius r₁ is between about 1 micron and3 microns; wherein the difference r₃−r₂ between said depressed trench'souter radius r₃ and said inner cladding's outer radius r₂ is betweenabout 3 microns and 7 microns; and wherein, for two turns around aradius of curvature of 5 millimeters at a wavelength of 850 nanometers,the optical fiber has bending losses of less than about 0.3 dB.
 11. Theoptical fiber according to claim 10, wherein$\frac{\sqrt{2\Delta}}{r_{1}}$ is about 5.4×10⁻³ μm¹ or greater. 12.The optical fiber according to claim 10, wherein$\frac{\sqrt{2\Delta}}{r_{1}}$ is about 5.9×10⁻³ μm⁻¹ or greater. 13.The optical fiber according to claim 10, wherein: said central core'souter radius r₁ is 36 microns or greater; and$\frac{\sqrt{2\Delta}}{r_{1}}$ is about 5.6×10⁻³ μm¹ or greater. 14.The optical fiber according to claim 10, wherein: said central core'souter radius r₁ is 37 microns or greater; and said depressed trench'svolume V₃ is −710%-μm² or greater.
 15. The optical fiber according toclaim 10, wherein: said central core's outer radius r₁ is 38 microns orgreater; and $\frac{\sqrt{2\Delta}}{r_{1}}$ is about 5.3×10⁻³ μm¹ orgreater.
 16. The optical fiber according to claim 10, wherein saidcentral core's outer radius r₁ is between about 39 microns and 41microns.
 17. The optical fiber according to claim 16, wherein saiddepressed trench's volume V₃ is −650%-μm² or greater.
 18. The opticalfiber according to claim 10, wherein: said central core's outer radiusr₁ is 40 microns or greater; and said depressed trench's volume V₃ is−830%-μm² or greater.
 19. The optical fiber according to claim 10,wherein the difference r₂−r₁ between said inner cladding's outer radiusr₂ and said central core's outer radius r₁ is less than 2 microns. 20.The optical fiber according to claim 10, wherein the difference r₃−r₂between said depressed trench's outer radius r₃ and said innercladding's outer radius r₂ is less than 5 microns.
 21. The optical fiberaccording to claim 10, wherein, for two turns around a radius ofcurvature of 5 millimeters at a wavelength of 850 nanometers, theoptical fiber has bending losses of less than about 0.2 dB.
 22. Anoptical system comprising the optical fiber according to claim 10.